Non-linear transduction results from the introduction of sufficiently intense, audio-modulated ultrasonic signals into an air column. Self-demodulation, or down-conversion, occurs along the air column resulting in the production of an audible acoustic signal. This process occurs because of the known physical principle that when two sound waves with different frequencies are radiated simultaneously in the same medium, a modulated waveform including the sum and difference of the two frequencies is produced by the non-linear (parametric) interaction of the two sound waves. When the two original sound waves are ultrasonic waves and the difference between them is selected to be an audio frequency, an audible sound can be generated by the parametric interaction.
Parametric audio reproduction systems produce sound through the heterodyning of two ultrasonic signals in a non-linear process that occurs in a medium such as air. The non-linearity of the medium results in the creation of audible sound produced in the medium that are the difference of the ultrasonic signals. Thus, two ultrasound signals that are separated in frequency result in a difference tone that is within the 20 Hz to 20,000 Hz range of human hearing.
The amount of power that can be used to drive an ultrasonic emitter is limited by a number of factors. These factors can include, for example, the amount of available power by the amplifier, the power level at which clipping occurs, the amount of power by which the emitters can be driven, and so on. Given these factors, the volume in which audio content can be played through an ultrasonic emitter is constrained to a maximum level. In order to increase output levels in ultrasonic audio systems, designers have typically relied on using higher power amplifiers and more robust or larger emitters. However, higher power amplifiers typically consume more energy and are more costly to produce, while more robust or larger emitters may also be more costly to produce and may suffer from other drawbacks such as poorer frequency response.